docs: def heightmap_get_normal(
      hm: np.ndarray, x: float, y: float, waterLevel: float
  ) -> Tuple[float, float, float]:
      """Return the map normal at given coordinates.
  
      Args:
          hm (numpy.ndarray): A numpy.ndarray formatted for heightmap functions.
          x (float): The x coordinate.
          y (float): The y coordinate.
          waterLevel (float): The heightmap is considered flat below this value.
  
      Returns:
          Tuple[float, float, float]: An (x, y, z) vector normal.
      """
      cn = ffi.new("float[3]")
      lib.TCOD_heightmap_get_normal(_heightmap_cdata(hm), x, y, cn, waterLevel)
      return tuple(cn)